The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for detecting an end-point during a chemical mechanical polishing operation.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a patterned stop layer, and planarizing the filler layer until the stop layer is exposed. For example, a conductive filler layer may be deposited on a patterned insulative stop layer to fill the trenches or holes in the stop layer. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing disk pad or belt pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
One way to determine the polishing endpoint is to remove the substrate from the polishing surface and examine it. For example, the substrate may be transferred to a metrology station where the thickness of a substrate layer is measured, e.g., with a profilometer or a resistivity measurement. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. This is a time-consuming procedure that reduces the throughput of the CMP apparatus. Alternatively, the examination might reveal that an excessive amount of material has been removed, rendering the substrate unusable.
More recently, in-situ optical monitoring of the substrate has been performed, e.g., with an interferometer or reflectometer, in order to detect the polishing endpoint. For example, when polishing a metal layer to expose an underlying insulative or dielectric layer, the reflectivity of the substrate will drop abruptly when the metal layer is removed. This drop can be detected to signal the polishing endpoint. Unfortunately, even when there is a sharp change in reflectivity, it may be difficult to determine the proper endpoint.
In one aspect, the invention is directed to an endpoint detection method for chemical mechanical polishing. In the method, a surface of a substrate is brought into contact with a polishing pad, and relative motion between the substrate and the polishing pad is created. A light beam is directed to contact the surface of the substrate, and the light beam is moved in a path across the substrate surface. An intensity signal produced by the light beam reflecting off the substrate is monitored, and a plurality of intensity measurements are extracted from the intensity signal as the light beam moves across the substrate. A first extreme intensity measurement is derived from the plurality of intensity measurements. The steps are iterated for a plurality of sweeps of the light beam across the substrate to generate a first plurality of extreme intensity measurements, and a polishing endpoint is detected based on the first plurality of extreme intensity measurements.
Implementations of the invention may include the following features. The first extreme intensity measurement is a maximum or minimum intensity measurement from the plurality of intensity measurements. A second extreme intensity measurement may be selected from the plurality of intensity measurements. The minimum intensity measurement for each iteration may be subtracted from the maximum intensity measurement from that iteration to create a plurality of differential intensity measurement. Detecting a polishing endpoint may include determining if criteria associated with either of the first or second plurality of extreme intensity measurements are satisfied. Alternatively, detecting a polishing endpoint may include determining if criteria associated with both the first and second plurality of extreme intensity measurements are satisfied. The substrate may include a filler layer, e.g., a metal layer, disposed over a stop layer, e.g., a dielectric layer, with the filler layer abutting the polishing pad. The polishing endpoint may indicate that the stop layer is at least partially exposed or that the stop layer is substantially exposed. An average intensity may be calculated from the plurality of intensity measurements for each iteration, and the polishing endpoint may be based on the average intensity measurements. The polishing pad may include a window, the light beam may be directed through the window, and the motion of the polishing pad relative to the substrate may cause the light beam to move across the substrate surface. A radial position for each intensity measurement may be determined. The intensity measurements may be divided into a plurality of radial ranges according to the radial positions. An extreme intensity measurement may be selected from the intensity measurements in each of the plurality of radial ranges. Polishing may be stopped at the polishing endpoint, or a polishing parameter, such as a polishing consumable, e.g., a slurry, may be changed at the polishing endpoint.
Advantages of the invention include one or more of the following. A wider range of endpoint detection algorithms are available, making the optical monitoring system useful in a wider range of polishing procedures. The endpoint detection procedure is more robust and lees likely to fail. Endpoint detection during metal polishing is improved. The polishing pressure, polishing speed, chemistry, and slurry composition may be altered when an underlying oxide layer is first exposed, and polishing may be stopped more precisely when the entire oxide and barrier layer have been removed.
Other features and advantages of the invention will to become apparent from the following description, including the drawings and claims.